A photodetector is a device converting a photo signal into an electronic signal and extensively used in various field, e.g. communication, computer engineering, controlling engineering and medical engineering, etc. However, different photodetectors are needed for different uses, and also are needed to be adapted according to the requirements of different photo signal frequencies. The detectable frequency band of conventional photodetectors is so narrow that a required photo response cannot be obtained when they are applied to a case needing a wide band or a frequency near the cut-off frequency of the used photodetector. This defect can be obviated by parallel connection of photodetectors with compensating frequency ranges.
The frequency range of a photodetector depends on the semiconductor material it uses. For example, silicon is used in a photodetector having a frequency range of 0.8.about.0.9 .mu.m, and a Ge, InGaAs or InGaAsP is used in a photodetector having a detectable frequency range of 1.0.about.1.6 .mu.m, etc. The reason why a photodetector using a specific semiconductor material detects specific frequency is that a semiconductor material with a specific band gap accepts a specific frequency band. Therefore, if a certain frequency or wavelength is needed, a photodetector with a certain semiconductor material is used; and if a wide frequency range is needed, several photodetectors with different semiconductor materials are used.
The shortcomings of the conventional photodetectors include the lack of flexibility in circuitry, assembly and application, and having troubles in the match among the voltage, frequency and circuit.
If a photodetector is able to sense light having a wavelength within a wide range, the inconvenience and high cost caused by using several different semiconductors to achieve the purpose will be reduced. Of course, the wider the wavelength range, the broader the application of the photodetector.